SVGHMI: add support for "enable expressions" with arbitrary variable name assignment.

HMI tree paths can be prefixed with a variable name "@name=/MY/HMI/VAR"
Widget declarations can end with a "#" followed by a JS expression that refers to name given to variables.
Widget is disabled if expression's result is false.

Commit includes some more-or-less related generated code refactoring, that should simplify extending widget's variables attributes.

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#!/usr/bin/env python

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# -*- coding: utf-8 -*-

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# This file is part of Beremiz

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# Copyright (C) 2019: Edouard TISSERANT

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#

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# See COPYING file for copyrights details.

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from __future__ import absolute_import

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from svghmi.svghmi import *